Levulinate esters, such as alkyl levulinates, are industrially relevant solvents or intermediates for the manufacture of pesticides, plasticizers, polymers or fuel additives. The acid-catalyzed ring opening of furfuryl alcohol in alcoholic media is a reaction of high interest yielding alkyl levulinates.
In contrast to the classical route involving 5-hydroxymethylfurfural (HMF) as a starting reagent or intermediate, alkyl levulinates can be produced from furfuryl alcohol through an elegant 100% atom economical process. Furthermore, furfuryl alcohol is industrially produced in large scale from the manufacture of hemicellulosic biomass. The selectivity of the catalytic production of alkyl levulinates from furfuryl alcohol is rather low due to the dominant acid-catalyzed side polymerization of furfuryl alcohol leading to an important formation of tar.
Document WO 2010/102203 discloses a method for the preparation of alkyl levulinates starting from furfuryl alcohol and an alkanol in the presence of a protic acid such as hydrochloric acid or sulfuric acid. A mixture alkyl levulinate/alkanol instead pure alkanol is used as a solvent for dilution of the furfuryl alcohol. The catalysts used in this document are generally not easily recyclable since tarry products are formed when the concentration of furfuryl alcohol is increased.
J. R. Kean and A. E. Graham, Catalysis Communications 59 (2015) 175-179, discloses a method for synthesizing alkyl levulinates from furfuryl alcohols using Indium(III) triflate catalysts. The amount of furfuryl alcohol introduced in the process of said document is relatively low.
To inhibit side polymerization reactions, previously reported homogeneous or heterogeneous catalytic processes are conducted under diluted conditions (furfuryl alcohol loading of 2-3 wt %).
Yao-Bing Huang et al. (Green Chem. 2016, 18, 1516-1523) discloses the conversion of furfuryl alcohol into alkyl levulinates catalyzed by metal salts comprising halogen or sulfated oxides, such as SnCl4 or AlCl3 catalysts. It has been observed that catalysts of type SnCl4 or AlCl3 provide the conversion with a relatively low productivity.
It was thus an object of the present invention to develop a process for converting furfuryl alcohol into levulinate esters with a catalytic system that permits to get very high selectivities and productivities.